1. Field of the Invention
The present invention relates to an oxynitride phosphor and a light emitting device using the same.
2. Description of the Related Art
In a field of general illumination, a solid-state illumination, specifically, a white light illumination using a semiconductor light emitting diode has attracted a lot of attention and thus researches and developments have been widely and vigorously performed.
Such a white light emitting diode lamp (light emitting device) has already acquired a luminous efficiency comparable or superior to that of an incandescent lamp, even though the diode lamp is still in the process of development. In the near future, the white light emitting diode lamp is thought to become widely used as energy-saving illumination equipment.
In addition, the white light emitting diode lamp is able to reduce an influence on environment since it is free of substances that impose a heavy burden on the environment, such as mercury or the like.
Moreover, the diode lamp has so small a size that it is quite often used as a backlight source of a liquid crystal display apparatus or the like and built-in in a cellular phone or the like.
The mainstream white light emitting diode lamp is fabricated by combining a blue light emitting diode chip and a Europium-activated Ca-alpha-SiAlON phosphor (See, for example, Japanese Patent Application Laid-open Publication No. 2002-363554 for the phosphor, and Japanese Patent Application Laid-open Publication No. 2003-124527 for the lamp).
FIG. 1 illustrates excitation and emission spectra of Ca0.88Si9.135Al2.865O0.955N15.045:Eu0.05 as an example of Eu2+-activated Ca-alpha-SiAlON phosphor that the inventors of the present invention synthesized for purposes of experiment.
This excitation spectrum was measured with an emission monitoring wavelength of 585 nm using a fluorescence spectrophotometer. The peak excitation wavelength is 449.6 nm.
This excitation spectrum has a wide excitation range in wavelengths shorter than 449.6 nm. Therefore, even when an excitation wavelength shifts toward shorter wavelength from the peak excitation wavelength, only a moderate reduction takes place in excitation efficiency.
On the other hand, a rather abrupt reduction in excitation efficiency takes place in wavelengths longer than 449.6 nm even if the excitation range therein is wide enough compared with that of other phosphors. In other words, the excitation spectrum has a steep slope in the range.
When the aforementioned phosphor is applied to a white light emitting diode lamp, its excitation spectrum is preferably as flat as possible. This is because the white light emitting diode lamp with a flat excitation spectrum is less sensitive to a wavelength shift that may be caused by parameter variations in fabricating a blue light emitting diode chip or temperature changes during use of the blue light emitting diode chip, or the like. Accordingly, only a slight change occurs in chromaticity of such a white light emitting diode lamp.
When the excitation spectrum has a steep slope, an emission intensity of the phosphor is sensitive to a wavelength shift of the excitation wavelength, accordingly leading to a large change in chromaticity of the white light emitting diode lamp.
Such a chromaticity change ends up in a reduction in fabrication yield due to chromaticity variations and causes an adverse influence during use.
Especially, since an emission spectrum of a blue light emitting diode chip generally shifts toward longer wavelengths with an increase in temperature, when the chip having a center emission wavelength of 450 nm that corresponds to the peak excitation wavelength of the phosphor is used, the excitation spectrum has preferably a gentle slope in longer wavelengths.
In other words, there has been desired an alpha-SiAlON of which emission efficiency is improved in wavelengths longer than the peak excitation wavelength in its vicinity.
Also, there has been desired a phosphor that realizes a wider chromaticity range than known phosphors and its chromaticity adjusting technology.
As for a light emitting device for general illumination, various light emitting devices having various color temperatures have been required depending on the application. In order to satisfy such a requirement, there has been desired various phosphors having various chromaticities.
Japanese Patent Application Laid-open Publication No. 2002-363554 discloses that a peak emission wavelength can be varied continuously in a range of from 560 nm to 590 nm by changing an activator amount of Eu2+. The phosphor disclosed in Japanese Patent Application Laid-open Publication No. 2003-124527 lies in a range of from 546 nm to 583 nm.
However, there has been required a phosphor that can be excited by blue light and emit light having a further longer wavelength than that of the phosphors disclosed in the above publications.
One example of such a phosphor is disclosed in R-J. Xie et al., “Eu2+-doped Ca-alpha-SiAlON: A yellow phosphor for white light-emitting diodes,” Applied Physics Letters, Vol. 84, Number 26, pp. 5404-5406 (2004). This phosphor has a chromaticity coordinates of (x, y) from (0.491, 0.497) to (0.560, 0.436) in the CIE1931 color specification system. This means that the dominant emission wavelength is in a range of from 578 nm to 588 nm.
However, regarding a source material for the above phosphor, a very expensive nitride as a source of a solid-solution element is required in addition to silicon nitride and aluminum nitride. For this reason, a longer wavelength phosphor that can be synthesized at a moderate price has been desired.
In view of the above circumstances, the present invention is directed at a provision of an oxynitride phosphor that can reduce chromaticity shifts and production costs, and a light emitting device using the same.